A conventional heat exchanger housed in a combustion device, such as a water heater and a heat source device for a room heater, includes a case body having an upper opening and a lower opening, a plurality of thin plate-shape heat transfer fins arranged side by side at predetermined intervals in the case body, and a plurality of heat-transfer tubes inserted through tube insertion holes provided in each of the heat transfer fins so as to penetrate the heat transfer fins in a direction perpendicular to each of the heat transfer fins. (For example, Japanese Unexamined Patent Publications No. 2011-231993 A and No. 2001-153468 A)
Conventionally, there has been known a heat transfer fin and a heat transfer tube made of highly thermally conductive stainless steel-based metal. The heat transfer fin and the heat transfer tube are bonded with a metallic brazing material (such as a nickel-based brazing material) having a lower melting point than that of the stainless steel-based metal.
FIG. 6A is a schematic front view showing a heat transfer fin (2) used in a conventional heat exchanger. As shown in FIG. 6A, a plurality of heat-transfer-tube insertion holes (20) are formed in a heat transfer fin (2). A brazing material holding portion (21) made by cutting out an upper end edge of the heat transfer fin (2) into a semi-circular shape is formed above each of the heat-transfer-tube insertion holes (20) positioned at the uppermost stage.
In a brazing step for bonding the heat transfer fin (2) and heat transfer tubes (30), first of all, the heat transfer tubes (30) are inserted through the heat-transfer-tube insertion holes (20) of the heat transfer fins (2) arranged side by side in a case body, so that a subassembly is manufactured. Subsequently, as shown in FIG. 6B, after a rod shape or paste brazing material (3) is placed on or applied to the brazing material holding portion (21), the subassembly is heated in a brazing furnace. The fluid brazing material (3) melted by heating then dribbles down to a top portion of an outer circumferential surface of the heat transfer tube (30) inserted through the heat-transfer-tube insertion hole (20) at the uppermost stage from the brazing material holding portion (21), and further flows down a gap between a circumferential edge of the heat-transfer-tube insertion hole (20) of the heat transfer fin (2) and the outer circumferential surface of the heat transfer tube (30) by capillary action. After that, when the brazing material (3) is solidified in a cooling step, the heat transfer tube (30) are brazed and fixed to the circumferential edge of the heat-transfer-tube insertion hole (20) of the heat transfer fin (2) with the brazing material (3).
In the brazing step, ideally, as shown in FIG. 6B, the heat transfer fin (2) is placed in the furnace while being kept in a horizontal posture in such a manner that a center of the heat-transfer-tube insertion hole (20) matches a center of the brazing material holding portion (21) on a vertical line in an up-and-down direction of the case body (not shown). When the heat transfer fin (2) is placed in the furnace in such an idealistic state, the brazing material (3) drops down to the top portion (31) of the outer circumferential surface of the heat transfer tube (30) from the brazing material holding portion (21) as shown by solid arrows in FIG. 6B, and further evenly flows down to the left and right side of the outer circumferential surface of the heat transfer tube (30) from the top portion (31) of the outer circumferential surface. With this configuration, the brazing material spreads over the substantially entire outer circumferential surface of the heat transfer tube (30). Thus, the heat transfer tubes (30) are stably brazed and fixed to the heat-transfer-tube insertion holes (20) of the heat transfer fin (2).
However, in an actual brazing step, as shown in FIG. 6C, the heat transfer fin (2) is placed in the furnace in a non-idealistic state that the heat transfer fin (2) is slightly inclined in the up-and-down direction. When the heat transfer fin (2) is inclined in the up-and-down direction, the brazing material (3) dropping down from the brazing material holding portion (21) flows down only to either the left or right side of the outer circumferential surface of the heat transfer tube (30) from a drop-down position as shown by dotted arrow in FIG. 6C. When the brazing material (3) unevenly flows down to the outer circumferential surface of the heat transfer tube (30) and then is solidified in the cooling step, only a partial region of the heat transfer tube (30) on one side where the brazing material (3) flows is brazed to the circumferential edge of the heat-transfer-tube insertion hole (20) of the heat transfer fin (2). As a result, there is a problem that brazing failure is caused.